Chapter 32 Notes: Microbial Infections
➢ 32.1 Many Types of Microbial Interactions Exist
The stable association of one organism with another is called symbiosis, regardless of whether
the association is beneficial for both partners
▪ Symbiosis: The living together or close association of two dissimilar organisms, each of
these organisms being known as a symbiont.
Symbionts can live within the host as an endosymbiont, or on the surface of the host as an
ectosymbiont. Finally, symbionts alter not only the health of their hosts, but their behavior,
evolution, and reproductive success as well
Mutualism defines the relationship in which some reciprocal benefit accrues to both partners.
This is an obligatory relationship in which the mutualist and the host are dependent on each
other. In many cases, the individual organisms will not survive when separated. Several
examples of mutualism are presented next.
▪ Mutualism: A type of symbiosis in which both partners gain from the association and are
dependent on each other.
▪ Mutualist: An organism associated with another in an obligatory relationship that is
beneficial to both.
Many marine invertebrates (sponges, jellyfish, sea anemones, corals, ciliated protists) harbor
endosymbiotic dinoflagellates called zooxanthellae within their tissues.
▪ Coral bleaching: The loss of photosynthetic pigments by either physiological inhibition
or expulsion of the coral photosynthetic endosymbiont, zooxanthellae, a dinoflagellate.
Ruminants are the most successful and diverse group of mammals on Earth today.
Examples include cattle, deer, elk, bison, water buffalo, camels, sheep, goats, giraffes, and
caribou. These animals spend vast amounts of time chewing their cud—a small ball of partially
digested grasses that the animal has consumed but not yet completely digested. It is thought that
ruminants evolved an “eat now, digest later” strategy because their grazing can often be
interrupted by predator attacks.
When the animal eats plant material, it is mixed with saliva and swallowed without
chewing to enter the rumen. Here, microbial attack and further mixing coats the grass with
microbes, reducing it to a pulpy, partially digested mass. At this point, the mass moves into the reticulum, where it is regurgitated as cud, chewed, and reswallowed by the animal. As this
process proceeds, the grass becomes progressively more liquefied and flows out of the rumen
into the omasum and then the abomasum. Here the nutrient-enriched grass meets the animal’s
digestive enzymes, and soluble organic and fatty acids are absorbed into the animal’s
The oxidation of these organic substrates, which results in the generation of H2 quickly
consumed by methanogens, is termed syntrophy. Syntrophy is an association in which the
growth of one organism either depends on, or is improved by, growth factors, nutrients, or
substrates provided by another organism growing nearby.
▪ Syntrophism: The association in which the growth of one organism either depends on, or
is improved by, the provision of one or more growth factors or nutrients by a neighboring
organism. Sometimes both organisms benefit.
For most microbial ecologists, the nonobligatory aspect between host and symbiont differentiates
cooperation from mutualism.
▪ Cooperation: A positive but not obligatory interaction between two different organisms.
▪ Commensalism: A type of symbiosis in which one individual gains from the association
(the commensal) and the other is neither harmed nor benefited.
Commensalism is a relationship in which one symbiont, the commensal, benefits, while the host
is neither harmed nor helped.
Commensal is the partner who benefits
2 Figure 32.9 Examples of Predatory Bacteria
Found in Nature.
(a) Vampirococcus has a unique epibiotic
mode of attacking a prey bacterium; and (b)
Daptobacter showing its cytoplasmic location
as it attacks a susceptible bacterium.
Parasitism: Microbes Harm the Host
Parasitism is one of the most complex microbial interactions; the line between parasitism and
predation is difficult to define. In both cases, the relationship between two organisms is
beneficial for one and harmful for the other. However, parasite and host must coexist, at least
temporarily, so the microbe has enough time to reproduce and colonize a new host. Coexistence
may involve nutrient acquisition, physical maintenance in or on the host, or both. But what
happens if the host-parasite equilibrium is upset? If the balance favors the host (perhaps by a
strong immune defense or antimicrobial therapy), the parasite loses its habitat and may be unable
to survive. On the other hand, if the equilibrium is shifted to favor the parasite, the host becomes
ill and, depending on the specific host-parasite relationship, may die.
▪ Parasitism: A type of symbiosis in which one organism benefits from the other and the
host is usually harmed.
▪ Parasite: An organism that lives on or within another organism (the host) and benefits
from the association while harming its host. Often the parasite obtains nutrients from the
Genomic Reduction occurs when the symbiont has become dependent on the host for specific
functions, such as synthesis of key metabolites. It is observed in the aphid endosymbiont
Buchnera aphidicola; it has also occurred in the human pathogens Mycobacterium leprae,
3 Mycoplasma genitalia, and Encephalitozoon cuniculi, a microsporidium. These microorganisms
now can only survive inside their host cells.
▪ Genomic Reduction: The decrease in genomic information that occurs over evolutionary
time as an organism or organelle becomes increasingly dependent on another cell or a
Competition arises when different organisms within a population or community try to acquire
the same resource, whether this is a physical location or a particular limiting nutrient. If one of
the two competing organisms can dominate the environment, whether by occupying the physical
habitat or by consuming a limiting nutrient, it will overtake the other organism. This
phenomenon was studied by E. F. Gause, who in 1934 described it as the competitive exclusion
▪ Competition: An interaction between two organisms attempting to use the same resource
(nutrients, space, etc.).
▪ Competitive exclusion principle: Two competing organisms overlap in resource